The endocrine response to prolonged dexamethasone treatment was investigated in six postmenopausal women with generalized mammary carcinoma. Plasma cortisol levels decreased rapidly and became undetectable whereas significant concentrations of plasma dehydroepiandrosterone and androstenedione persisted throughout the study, even in two ovariectomized patients, indicating a certain degree of autonomy or a greater resistance of adrenal 'androgens' to the inhibition of ACTH secretion. Except in the ovariectomized patients, plasma testosterone did not fall significantly whereas the plasma oestrogens tended progressively towards undetectable concentrations. A similar response was found in six normal postmenopausal women although the disappearance of their oestrogens was relatively rapid. This indicates that much of the testosterone present after the menopause could still be produced by the ovaries whereas the ovarian production of oestrogens becomes negligible. The delayed disappearance of oestrogens in the patients with mammary carcinoma indicates that the persisting adrenal 'androgens' remained efficient precursors of oestrogen synthesis within the peripheral tissues and presumably within the mammary tumour itself. Plasma dihydrotestosterone behaved like the plasma oestrogens. Despite the fall in plasma oestrogens, plasma gonadotrophins did not increase further but plasma prolactin rose progressively. The persistance of steroid sex hormones and the rise of plasma prolactin might explain the poor response to dexamethasone treatment in mammary carcinoma.